1. Field of the Invention
The invention relates generally to electrically driven centrifugal submersible downhole pumps, and in particular to system of using complementary compressive devices to prevent diffuser rotation.
2. Description of the Related Art
When an oil well is initially completed, the downhole pressure may be sufficient to force the well fluid up the well tubing string to the surface. The downhole pressure in some wells decreases, and some form of artificial lift is required to get the well fluid to the surface. One form of artificial lift is suspending an electric submersible pump (ESP) downhole, normally on the tubing string. The ESP provides the extra lift necessary for the well fluid to reach the surface. One type of ESP is a centrifugal pump. Centrifugal pumps have a series of impellers inside of a tubular housing, which are rotated by a drive shaft in order to propel fluids from the radial center of the pump towards the tubular housing enclosing the impellers.
The impellers have an inlet or an eye towards the radial center portion around the drive shaft. Spinning the impeller creates centrifugal forces on the fluid in the impeller. The centrifugal forces increase the velocity of the fluid in the impeller as the fluid is propelled towards the tubular housing.
The height that the fluid would be able to travel in a passageway extending vertically from the exit of the impeller is the head generated from the impeller. A large amount of head is necessary in order to pump the well fluid to the surface. Either increasing the impeller diameter or increasing the number of impellers can increase the amount of head generated by a pump. The diameter of the impellers is limited by the diameter of the well assembly. Therefore, increasing the number of impellers is the common solution for downhole pumps in order to generate enough head to pump the well fluid to the surface.
The fluid enters a stationary diffuser after exiting the impeller. The fluid loses velocity in the diffuser because it is stationary. Decreasing the velocity of the fluid in the diffuser causes the pressure of the fluid to increase. The diffuser also redirects the fluid to the eye or inlet of the next impeller. Each impeller and diffuser is a stage in a pump. The pressure increase from one stage is additive to the amount of head created in the next stage. After enough stages, the cumulative pressure increase on the well fluid is large enough that head created in the last impeller pumps the well fluid to the surface.
Each impeller mounts directly to the drive shaft, but the diffusers slide over the drive shaft and land on the diffuser of the previous stage. A pre-load is applied so that this contact between the diffusers creates a large enough frictional force to prevent the diffusers from spinning with the drive shaft. Under some operating conditions, the temperature of the well fluid is cool enough to cause the material of the diffusers to shrink. Shrinkage in the material of the diffusers may cause gaps to form between diffuser interfaces and a loss in the frictional resistance to spin.
The pressure does not increase between stages when the diffusers are not stationary. The pump will not be capable of generating enough head to pump the well fluid to the surface when each stage does not increase the pressure. Furthermore, if the diffusers do not allow the fluid velocity to decrease between stages, the intake velocity of the fluid in the next stage may cause serious performance problems. These problems can cause drive shaft vibrations large enough to damage the pump. Methods of applying pre-loads to the diffusers are known in the art, but the known methods cannot provide additional forces on the diffusers in case the temperature of the fluid causes the diffusers to shrink during operation.
A first compressive device provides a pre-compressive load on the stack of diffusers. This pre-compressive load prevents the diffusers from rotating with the drive shaft and impellers by insuring that the resistance to slippage at diffuser interfaces is too large for the drive shaft and impellers to overcome. A spring member provides additional compressive forces on the stack of diffusers. The pre-compressive load from the first compressive device holds the diffusers stationary relative to the rotating shaft and impellers under normal operating conditions. The spring member becomes the primary compressive force only at times when the first compressive device fails to compress the stack of diffusers.
Such an event may occur when the material of the diffusers shrinks because the diffusers cool down when in contact with well fluid that is cooler in temperature than normal. The shrinkage in the dimensions of the diffusers causes gaps to form between the diffusers. The first compressive device cannot be adjusted to a different position during operation. Therefore, there is no frictional force resisting slippage because of the gaps at the interfaces of the diffusers. Accordingly, the diffusers could rotate with the impellers and drive shaft. However, the spring member provides an additional force that can continue to apply a compressive load on the stack of diffusers even after cooler well fluid causes the diffusers to shrink. The compressive force from spring member closes any gaps that would form, and insures that there is a slip resisting frictional force at the diffuser interfaces.
Preventing any formation of gaps between diffusers with the spring member prevents the diffusers from rotating with the drive shaft and impellers. Having stationary diffusers allows the velocity of the well fluid to decrease, which in turn causes the pressure of the well fluid to increase. Decreasing the well fluid velocity and increasing the well fluid pressure is necessary for generating the necessary head to pump the well fluid to the surface. Furthermore, diffuser rotation accelerates wear which increases the running clearances between the diffusers and impellers, which may damage the pump. Preventing diffuser rotation helps to maintain the running clearances between the impellers and diffusers, thereby protecting the pump from damage. By maintaining the design operating conditions and running clearance of the pump, the spring member prevents cooler temperature well fluids from damaging the pump, and the spring member prevents production disruptions by maintaining well fluid flow to the surface.